Among image display apparatus using a liquid crystal display device or the like, an image display apparatus has been disclosed for example in Japanese patent applications laid open No. 6-324320 and No. 7-7704 in which resolution of the liquid crystal display device is improved by effecting a pixel shifting operation called wobbling where the optical axis of light from the liquid crystal display device is wobbled in predetermined directions.
A description will now be given with respect to the general construction of an image display apparatus in which resolution is improved by such optical wobbling operation. As shown in FIG. 1, a back light 102 for emitting white light is placed on the back side of a color liquid crystal display device 101, and a wobbling device (a pixel shifting unit) 103 for wobbling in predetermined directions the optical axis of light from the color liquid crystal device 101 is placed on the front side of the color liquid crystal display device 101. Here, odd field images and even field images of input video signal are displayed on the color liquid crystal display device 101 at the same pixels thereof through an image display control circuit 104. In accordance with their display timing, the optical axis of light from the color liquid crystal display device 101 is wobbled in predetermined directions by the wobbling device 103.
The wobbling device 103 includes a polarization changing liquid crystal plate 105 and a birefringence plate 106 which is placed on the front side thereof. Here, ON/OFF of voltage across the polarization changing liquid crystal plate 105 is controlled by a wobbling liquid crystal drive circuit 107 based on synchronizing signal of the video signal to be displayed on the color liquid crystal display device 101. The light from the color liquid crystal display device 101 is thereby transmitted without changing its polarization when the voltage is ON, while, when the voltage is OFF, the light from the color liquid crystal display device 101 is transmitted with changing its polarization through 90 degrees, effecting the wobbling operation by changing the location to be emitted from the birefringence plate 106 in accordance with such direction of polarization. It should be noted that, since the color liquid crystal display device 101 retains the image of the preceding field until rewriting of the image of the next field, one of the electrodes of the polarization changing liquid crystal plate 105 is divided into parts each with a plurality of lines such as 5 lines. The other electrode is used as a common electrode and application of voltage is controlled by selecting the one of the electrodes in accordance with the timing of line scan of the color liquid crystal display device 101.
The following operation is performed when alternately displaying odd field images and even field images on the color liquid crystal display device 101. In particular, a case is supposed here as shown in FIG. 2A that the horizontal pixel pitch is Px and the vertical pixel pitch is Py of a pixel group in delta array of the color liquid crystal display device 101. An oblique wobbling operation of 0.75 Px in the horizontal direction and 0.5 Py in the vertical direction, for example, is performed by the above described wobbling device 103 so that the pixel array of the color liquid crystal display device 101 is located at the position as indicated by the broken lines in FIG. 2B when an odd field image is to be displayed, while the pixel array is located at the position indicated by solid lines when an even field is to be displayed. Specifically, if for example Px is 18 μm and Py is 47.5 μm, the wobbling operation is effected so as to achieve an oblique distance of about 27.3 μm, shifted by 13.5 μm horizontally and 23.75 μm vertically.
For this reason, a crystallographic axis 106a of the birefringence plate 106 is set as shown in FIG. 3 in a direction inclined with respect to the XY coordinate of on the color liquid crystal display device surface and Z direction which is normal thereto. Here, when the direction of polarization of incidence agrees with the direction of polarization of light from the color liquid crystal display device, the light from the color liquid crystal display device is transmitted as extraordinary rays so as to shift the pixels. When the direction of polarization of incidence is rotated through 90 degrees with respect to the direction of polarization of light from the color liquid crystal display device, it is transmitted intact as ordinary rays without shifting the pixels.
In this manner, as shown in FIG. 4, when the image of an odd field is to be displayed on the color liquid crystal display device 101, voltage application to the region of the polarization changing liquid crystal plate 105 corresponding to the horizontal lines to be rewritten is turned ON, so as to transmit the light from such lines intact without rotating the direction of polarization through 90 degrees. The light is emitted by the birefringence plate 106 as extraordinary rays to shift the pixels. On the other hand, when the image of an even field is to be displayed, voltage application to the region of the polarization changing liquid crystal plate 105 corresponding to the horizontal lines to be rewritten is turned OFF, so as to transmit the light from the lines as rotated in the direction of polarization through 90 degrees, causing the birefringence plate 106 to emit the light intact as ordinary rays without shifting the pixels.
In addition, an image display apparatus is known to be provided with two units of such one-dimensional two-point pixel shifting unit each having a polarization changing liquid crystal plate and birefringence plate which are combined into a laminate where one of the units is rotated through 90 degrees about the axis of incident light with respect to the other so that a high resolution by two-dimensional four-point pixel shifting is achieved by performing four times of pixel shift in the vertical and horizontal directions within one frame or one field.
Further, though not related to high-resolution display using a pixel shifting unit, a field sequential color display apparatus having a construction as will be described below is disclosed in Japanese patent application laid open No. 8-248382. Specifically, the field sequential color display apparatus uses a monochrome CRT and a liquid crystal shutter (color filter) consisting of π-cell and color polarizing plate. To reduce color mixture that occurs between fields due to delay in the response speed of π-cell contained in the liquid crystal shutter, the switching timing of ON/OFF of the liquid crystal shutter is set within a blanking period occurring between two image periods of sequential color signals and the switching timing of ON/OFF is set so as not to cause color mixture.
If an attempt is made to display a high-resolution color image by applying the above described pixel shifting unit to a field sequential color display apparatus for effecting color displaying by switching among trichromatic images according to time, the following problem occurs. In particular, though there is not much problem when the liquid crystal cell is turned ON from OFF, the problem occurs when it is turned OFF from ON that pixels are seen to be displayed simultaneously at two locations in such transition period, i.e., at the original pixel location and at the pixel location shifted by pixel shifting. A color leakage is thereby caused.
In the above laid-open publication, a disclosure is made merely with respect to color mixture in the field sequential color display apparatus using a liquid crystal shutter consisting of π-cell and color polarizing plate as the color filter. It fails to mention the occurrence of color leakage in a high-resolution field sequential color display apparatus using a pixel shifting unit.
To eliminate the above problems in achieving a display at high resolution by using a pixel shifting unit in field sequential color display apparatus, it is an object of the invention to provide a field sequential color display apparatus in which sensory color mixture due to color leakage arising from the shifting of pixel can be reduced.